Observations of high-frequency acoustic attenuation due to bubble entrainment at estuarine fronts

Frontal boundary features with strong gradients in water properties and velocity are common features in coastal environments where fresh discharge meets ambient ocean waters. Multiple mechanisms related to convergence at these fronts support the entrainment of bubbles. First, horizontal velocity gradients cause surface waves to steepen and break at these boundaries. Second, localized downwelling can further entrain and transport these bubbles. In experiments using broadband echosounders (45–420 kHz) at the Connecticut River ebb plume front and James River tidal intrusion front, bubble entrainment was observed to cause frequency-dependent excess attenuation. Seabed backscattering near fronts is used as a baseline to identify attenuated echoes from the seabed under bubble plumes near the fronts. Despite observing significant (>10 dB in some cases) excess attenuation in lower-frequency channels, we hypothesize that these fronts are unlikely to significantly affect propagation in the horizontal at these frequencies and that factors, such as refraction and three-dimension structure, are more operationally significant.